[Varicella vaccine effectiveness in schoolchildren in outbreaks in a semi-urban area]

Objective: To estimate the effectiveness of the varicella vaccine in school outbreaks during the academic year 2009-2010.

Material and method: Retrospective cohort study in public day-care centers and/or schools in an area in the region of Murcia. Spain. The participants were all children of 1 to 12 years who were in the same classroom where there was an outbreak of varicella. The main measurements were the sociodemographic, clinical and vaccination data, as well as variables related to varicella disease obtained through a questionnaire self-completed by parents, and from the computerized regional immunization registry (VACUSAN).

Results: A total of 51 varicella outbreaks were detected, with a median of 3 cases per outbreak at both educational levels. The overall vaccination coverage was 10.7% (95% CI 8.63 to 13.18), always being higher in Spanish schoolchildren versus foreign (OR: 21.21, 95% CI: 2.92 to 153.92, P<.001). Discrepancies were found between the vaccine questionnaire data and vaccination program (kappa=0.50, 95% CI: 0.43 to 0.58, P<.001). According to VACUSAN, the overall attack rate was 59.7 (95% CI: 55.82 to 63.43) in unvaccinated and 6.5 (95% CI: 2.54 to 15.45) in vaccinated children. An overall effectiveness of 89.1% (95% CI: 74.55 to 95.35) and 100% was obtained for 1 and 2 doses of vaccine, respectively.

Conclusions: There is a high effectiveness of varicella vaccine, emphasizing that the administration of two doses of vaccine produces an adequate and optimal protection against varicella disease. A discrepancy was found between the information provided by parents and official records. Finally, there was a lower vaccination coverage in the immigrant community.